The use of lubricants in heat transfer devices of the mechanical vapor recompression type, including refrigerators, heat pumps and air conditioning systems, is well known. In such devices, a working fluid of suitable boiling point evaporates at low pressure taking heat from the surrounding zone. The resulting vapor is then compressed and passed to a condenser where it condenses and gives off heat to a second zone. The condensate is then returned through an expansion valve to the evaporator, so completing the cycle. The mechanical energy required for compressing the vapor and pumping the fluid is provided by, for example, an electric motor or an internal combustion engine. As is the case with other mechanical equipment, it is necessary for the moving parts of the heat transfer devices to be adequately lubricated and the devices are generally designed to use lubricants which are miscible with the working fluids.
In the past, the heat transfer fluids typically used in these heat transfer devices were chlorofluorocarbons, such as dichlorodifluoromethane, together with a lubricant such as mineral oil. Today, the production of several of these chlorofluorocarbons is being severely limited by international agreement in order to protect the stratospheric ozone layer.
Unfortunately, some of the compounds, such as, for example, 1,1,1,2-tetrafluoroethane, which are being proposed as working fluids to replace dichlorodifluoromethane, are insufficiently miscible with mineral oils to allow the latter to be used as lubricants with these working fluids. Polyalkylene glycols have been proposed as alternative lubricants, but they are not entirely satisfactory for a number of reasons. For example, while polyalkylene glycols exhibit desired reverse solubility above a certain temperature such that the working fluid tends to separate from the lubricant, for example at temperatures above about 40.degree. C., they often attract water in an amount sufficient to cause corrosion in the equipment. More specifically, monols which rely upon ethylene oxide content to achieve sufficient miscibility and viscosity to permit their use as lubricants, such as is disclosed in European Patent application 336,171 assigned to Nippon Oil Co., Ltd., and polypropylene glycol diols and triols such as is disclosed in U.S. Pat. No. 4,755,316 assigned to Allied-Signal Inc. all tend to attract more water than might be desired. Furthermore, these compounds often fail to wet metals sufficiently to provide efficient lubrication and have an adverse effect on aluminium commonly used for the construction of compressors.
The discovery of new lubricants for hydrofluorcarbon working fluids that do not suffer from the water affinity problems and the lack of wetting efficacy associated with known polyalkylene glycols, when utilized in a fluorocarbon working fluid, would be highly desired by the heat transfer fluids community. Heretofore, an entirely satisfactory solution to those problems has not been proposed to the knowledge of the present inventors.